Magnetic circuit and method and apparatus for the manufacture thereof

ABSTRACT

A magnetic circuit includes a cylindrical yoke member having an inner circumferential surface and a collar formed at one end thereof and turned back substantially at right angles, and a plurality of permanent magnet segments of a partially circular arc in cross section having respective outer surfaces attracted magnetically to the inner circumferential surface of the cylindrical yoke member and respective one end faces attracted magnetically to the collar of the cylindrical yoke member. The magnetic circuit is manufactured by a method including the steps of magnetically attracting the permanent magnet segments to an outer peripheral surface of a center rod, causing the magnet segments in a magnetically attracted state to face the yoke member, using a nonmagnetic supporter to regulate the magnet segments in position, extracting the center rod from the magnet segments in a position-regulated state, and magnetically attracting and fixing the magnet segments to the yoke member. The magnetic circuit is applied to an actuator or a speaker.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic circuit for use in amagnet-aided actuator for converting electric energy to kinetic energyetc. and a method and apparatus for the manufacture thereof and to animprovement in a method for incorporating a permanent magnet into a yokemember. It further relates to an actuator or speaker using the magneticcircuit.

2. Description of the Prior Art

For various kinds of office automation equipment, linear motors (voicecoil motors), controllers, etc., for example, actuators of a variety oftypes have been developed. Concrete examples include an actuator havinga magnetic coil and a magnetic core made movable and an actuator havinga permanent magnet movable. In either case, the actuator comprises ayoke, a magnetic coil and a permanent magnet in combination.

Actuators using a permanent magnet have encountered problems as to howcontrol precision is to be made high and how impellent is to beenhanced. Attempts have been made to solve the problems throughmodifications of the structures of the actuators. As a high-precisioncontrollable actuator, for example, there has been proposed a linearactuator comprising an outer yoke and an inner yoke each having amagnetic coil wound around it and a cylindrical permanent magnetmagnetized in the radial direction (refer, for example, to JP-A HEI6-284670). A linear actuator comprising a side yoke, a first permanentmagnet of a given polarity and a second permanent magnet of the oppositepolarity, has been proposed as an actuator capable of enhancing theimpellent (refer, for example, to JP-A HEI 5-49226).

The former prior art adopts the configuration in which the pair of outeryoke and inner yoke each having a magnetic drive coil would around itare coaxially disposed using a spacer, the cylindrical permanent magnetsupported on a magnet holder and magnetized in the radial direction isdisposed as a movable member between the magnetic drive coils, and anoutput shaft piecing through the axis portion of the inner yoke isconnected to the magnet holder. According to this prior art, thisconfiguration makes it possible to provide a linear actuator exhibitinghighly precise controllability and enabling miniaturization andlightweight and higher versatility.

In the latter prior art, a hollow cylindrical permanent magnet isaffixed onto the inner circumferential surface of the side yoke formedin a bottomed hollow cylindrical shape from a ferromagnetic materialand, at the same time, a cylindrical center yoke made from aferromagnetic material is disposed coaxially with the side yoke on thebottom of the side yoke in a projecting manner, thereby configuring amagnetic circuit. This prior art has a structure in which an axiallymovable member is disposed in a magnetic void defined between thepermanent magnet and the center yoke. The permanent magnet comprises thefirst permanent magnet extending from the bottom of the side yoke to theneighborhood of an opening and the second permanent magnet magnetized tohave a polarity opposite to that of the first permanent magnet andprovided in the vicinity of the opening. By providing a drive coil and adetection coil, with the movable member facing the first and secondpermanent magnets respectively, it is made possible to materialize alinear actuator small in size, light in weight, enhanced in impellentand high in linearity and reliability.

While each of the former and latter prior art references uses acylindrical permanent magnet in a magnetic circuit constituting anactuator, a rare earth sintered magnet exhibiting high magneticintensity is formed into a cylindrical permanent magnet only with greatdifficulty from the manufacturing point of view.

Generally, a rare earth sintered magnet is produced by the powdermetallurgy method comprising the steps of compact-molding raw alloymaterial in a magnetic field into a cylindrical shape and subjecting thecompacted body to sintering and aging treatments. In this case, thecompacted body is liable to be shrunk and deformed during the process ofthe sintering treatment. When such shrinkage arises, circularity islowered to require polishing for adjustment of the outside and insidediameters before incorporating the magnet into the actuator, thusinducing an increase in man-hour and manufacturing cost. When the shapeof the magnet is made cylindrical, since the volume relative to theweight becomes large, the number of the magnets to be treated at a timewill be decreased to increase the sintering cost.

Though a difference is made depending on the structure of a magneticcircuit, where the dimensional accuracy in outside and inside diametersof a cylindrical rare earth sintered magnet is low when it is intendedto affix the magnet to a yoke, the area of the magnet in contact withthe yoke becomes substantially very small. As a result, the force ofaffixing the magnet to the yoke is considerably diminished to possiblynecessitate concomitant use of affixation by an adhesive. Furthermore, aproblem will arise when magnetizing the magnet. Specifically, amagnetizing apparatus large in size has to be used for the cylindricalmagnet when magnetizing the magnet either before or after the magnet isaffixed to the yoke.

Also in addition thereto, a cylindrical permanent magnet used in amagnetic circuit of an actuator generally has an orientation in thedirection of diameter (a so-called radial orientation). A cylindricalrare earth sintered magnet having the radial orientation, when beingformed in a small wall thickness, for example, sustains cracks or chipsduring the process of sintering, resulting in tendency to greatly lowerthe yield. This decrease in yield constitutes a serious cause ofpreventing an actuator from having high performance and from beingminiaturized.

In view of the conventional state of affairs described above, thepresent invention has been proposed. An object of the present inventionis to provide a magnetic circuit capable of materializing itshigh-performance property and miniaturization property withoutencountering any difficulty during the course of the manufacture thereofeven when a rare earth sintered magnet is used as a permanent magnet.Another object of the present invention is to provide a method andapparatus for efficiently manufacturing the magnetic circuit. Stillanother object of the present invention is to provide an actuator and aspeaker each having the magnetic circuit incorporated therein to attainits high-performance property.

SUMMARY OF THE INVENTION

To attain one of the above objects, the present invention provides amagnetic circuit comprising a cylindrical yoke member having an innercircumferential surface and a collar formed at one end thereof andturned back substantially at right angles, and a plurality of permanentmagnet segments of a partially circular arc in cross section havingrespective outer surfaces attracted magnetically to the innercircumferential surface of the cylindrical yoke member and respectiveone end faces attracted magnetically to the collar of the cylindricalyoke member.

To attain another object, the present invention provides a method forthe manufacture of a magnetic circuit, comprising the steps ofmagnetically attracting a permanent magnet segment of a partiallycircular arc in cross section to an outer periphery of a center rod,causing the magnet segment in a magnetically attracted state to face ayoke member, using a nonmagnetic supporter to regulate the magnetsegment in position, extracting the center rod from the magnet segmentin a position-regulated state and magnetically attracting and fixing themagnet segment to the yoke member.

An apparatus for the manufacture of a magnetic circuit, comprising acenter rod formed of a magnetic material and having an outer peripheralsurface to which a permanent magnet segment of a partially circular arcin cross section is attracted and a nonmagnetic supporter assuming aring shape and disposed around the outer peripheral surface of thecenter rod and coaxially with the center rod, wherein the center rod isinsertable into and detachable from the nonmagnetic supporter.

First of all, the fundamental idea of the present invention lies in notuse of a cylindrical permanent magnet, but use of permanent magnetsegments having a partially circular arc in cross section (hereinafterreferred to as C-shaped permanent magnet segments) into which acylindrical shape is divided at a predetermined angle and a combinationof the permanent magnet segments into a substantially cylindrical shape.

The C-shaped permanent magnet segments are easy to manufacture even inthe case of using rare earth sintered magnets. Since the segments formedeven in a small thickness sustain few cracks or chips during the processof sintering, they can be manufactured at high yield. Deformationthereof during the process of sintering can be suppressed to a greatextent as compared with cylindrical magnets. Also, since the C-shapedpermanent magnet segments can be stacked in their thickness direction,they can be magnetized in a lump to shorten the magnetization time to agreat extent.

These C-shaped permanent magnet segments are magnetically attracted tothe yoke member to configure the magnetic circuit of the presentinvention. Mere magnetic attraction of the permanent magnetic segmentsto the inner circumferential surface of a cylindrical yoke, for example,still brings to anxiety about the state in which the permanent magnetsegments have been fixed to the yoke. In the present invention,therefore, the yoke is provided at one end thereof with a collar whichis turned back substantially at right angles and to which the end facesof the permanent magnet segments are to be magnetically attracted. Withthis, the segments become in the firmly fixed state even without use ofan adhesive agent and, and the entire structure thereof is strong enoughto endure vibration resulting from its fall etc. Using rare earthmagnets, particularly R-T-B-based rare earth sintered magnets to bedescribed later, in the present invention proves to be effective becauseof very high magnetic intensity.

On the other hand, in the magnetic circuit using the C-shaped permanentmagnet segments, in order to form a cylindrical magnet, for example, aplurality of C-shaped permanent magnet segments have to be combined toincrease the number of magnet segments to be treated and possibly makethe incorporation thereof into a yoke member cumbersome and complicated.Since the magnet segments are attracted to or repelled by each other, itis hard enough to dispose the segments accurately in position and incombination.

In the present invention, therefore, the incorporation of the permanentmagnetic segments into the yoke member is implemented utilizing themagnetic attraction to the center rod and the release of the magneticattraction resulting from the extraction of the center rod. The presentinvention adopts the following procedure. To be specific, a center rodformed of a magnetic material is set in position. C-shaped permanentmagnet segments magnetized in advance in the thickness direction, forexample, are magnetically attracted to the peripheral surface of thecenter rod. When combining the C-shaped permanent magnet segments into acylindrical shape, for example, plural C-shaped permanent magnetsegments are attached to the peripheral surface of the center rod and,in this state, the C-shaped permanent magnet segments are caused to facethe inner circumferential surface of a yoke member. Subsequently, anonmagnetic support is used to regulate the positions of the C-shapedpermanent magnet segments without being moved and the center rod isextracted. As a result, the magnetic attraction exerted between theC-shaped permanent magnet segments and the center rod is released tobring the C-shaped permanent magnet segments to a free state. At thistime, the magnetic attraction is exerted between the yoke member formedof a magnetic material and the C-shaped permanent magnet segments toattract the C-shaped permanent magnet segments to the innercircumferential surface of the yoke member and fixed thereto by means ofthe magnetic attraction.

By the simple operation of the magnetic attraction of the C-shapedpermanent magnet segments to the center rod and the insertion of thecenter rod into and extraction thereof from the yoke member, pluralC-shaped permanent magnet segments can be accurately positioned andmagnetically attracted and fixed to the yoke member. Therefore,insertion of the magnet segments into the yoke member can continuouslybe performed for a short period of time and, after the insertion, it isunnecessary to magnetize the magnet segments.

The magnetic circuit of the present invention having the configurationmentioned above is applicable to actuators or speakers, for example, tomaterialize high-performance actuators or speakers. That is to say, theactuator of the present invention generates an actuating force by meansof the magnetic circuit and a drive coil in cooperation with each other.The magnetic circuit is configured such that the plural permanent magnetsegments of a partially circular arc in cross section are magneticallyattracted to the inner circumferential surface of the cylindrical yokemember having at one end thereof the collar turned back substantially atright angles and such that the end faces of the plural magnet segmentsare magnetically attracted to the collar.

The speaker of the present invention vibrates a diaphragm joined to avoice coil by means of the magnetic circuit and the voice coil incooperation with each other. The magnetic circuit is configured suchthat the plural permanent magnet segments of a partially circular arc incross section are magnetically attracted to the inner circumferentialsurface of the cylindrical yoke member having at one end thereof thecollar turned back substantially at right angles and such that the endfaces of the plural magnet segments are magnetically attracted to thecollar.

According to the magnetic circuit of the present invention, since not acylindrical permanent magnet, but the C-shaped permanent magnet segmentsare incorporated, difficulty in producing a cylindrical permanentmagnet, particularly a cylindrical rare earth sintered magnet, can beeliminated. Also, use of the magnetic circuit enables an inexpensive andhigh-performance actuator or speaker to be materialized. Furthermore,since the C-shaped permanent magnet segments are magnetically attractedto both the inner circumferential surface and the turned-back collar ofthe yoke member, it is made possible to fix the segments to the yokemember without use of an adhesive agent. Thus, it is possible to providea magnetic circuit strong enough to endure vibration resulting from itsfall etc at low cost.

On the other hand, according to the manufacturing method and apparatusof the present invention, since the plurally divided C-shaped permanentmagnet segments can easily be incorporated into the yoke member, it ismade possible to manufacture a high-performance and miniaturizedmagnetic circuit with high efficiency. Also, since not a cylindricalpermanent magnet, but the C-shaped permanent magnet segments areincorporated, difficulty in producing a cylindrical permanent magnet,particularly a cylindrical rare earth sintered magnet, can beeliminated. Thus, it is possible to provide an inexpensive magneticcircuit with high productivity.

The above and other objects, characteristic features and advantages willbecome apparent to those skilled in the art from the description to begiven herein below with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a schematic perspective view showing one example of amagnetic circuit and an actuator having the magnetic circuitincorporated therein according to the present invention and FIG. 1( b) aschematic cross section thereof.

FIG. 2 is a schematic perspective view illustrating the manner in whichC-shaped permanent magnet segments are being magnetized.

FIG. 3 is a schematic perspective view illustrating the scheme of themanufacturing apparatus and method according to the present invention.

FIG. 4 illustrates a process of inserting the C-shaped permanent magnetsegments into a yoke member according to the manufacturing apparatus andmethod of the present invention, FIG. 4( a) being a schematicperspective view illustrating the state in which the C-shaped permanentmagnet segments are being supplied onto a central rod, FIG. 4( b) beinga schematic perspective view illustrating the state in which theC-shaped permanent magnet segments have been attracted onto the centralrod, FIG. 4( c) being a schematic perspective view illustrating thestate in which the central rod has been inserted into the yoke member,and FIG. 4( d) being a schematic perspective view illustrating the statein which the central rod has been extracted from the yoke member and inwhich the C-shaped permanent magnet segments have been set in positionwithin the yoke member.

FIG. 5 illustrates in detail an operation of setting the C-shapedpermanent magnet segments in position within the yoke member, FIG. 5( a)being a schematic cross section illustrating the state in which theC-shaped permanent magnet segments face the inner circumferentialsurface of the yoke member, FIG. 5( b) being a schematic cross sectionillustrating an operation of extracting the central rod, and FIG. 5( c)being a schematic cross section illustrating the state in which theC-shaped permanent magnet segments have been attracted to the yokemember.

FIG. 6 illustrates one example of the manufacturing apparatus accordingto the present invention provided with four sets of central rods eachequipped with a nonmagnetic supporter, FIG. 6( a) being a schematicperspective view illustrating the state in which the C-shaped permanentmagnet segments have been set in position onto each central rod and FIG.6( b) being a schematic perspective view illustrating the state in whichthe C-shaped permanent magnet segments have been set in position withineach yoke member.

FIG. 7 illustrates modifications of the present invention, FIG. 7( a)being a schematic cross section illustrating the state in which theC-shaped permanent magnet segments have been inserted into a yoke memberhaving a return part, FIG. 7( b) being a schematic cross sectionillustrating the state in which the C-shaped permanent magnet segmentshave been inserted onto the inner circumferential of a yoke member, andFIG. 7( c) being a schematic cross section illustrating the state inwhich the C-shaped permanent magnet segments have been inserted into ayoke member having concaves for accommodating an adhesive therein.

FIG. 8 is a schematic perspective view illustrating an example of theapparatus for setting the C-shaped magnet segments stored in amagnetizing jig in position onto each central rod.

FIG. 9 illustrates the procedure of setting the C-shaped magnet segmentsin position in the apparatus shown in FIG. 8, FIG. 9( a) being aschematic front view illustrating an operation of setting a first set ofC-shaped magnet segments in position, FIG. 9( b) being a schematic frontview illustrating an rotation operation of the central rod, and FIG. 9(c) being a schematic front view illustrating an operation of setting asecond set of C-shaped magnet segments in position.

FIG. 10 is a schematic cross section showing one example of a speakerhaving the magnetic circuit of the present invention incorporatedtherein.

FIG. 11 is a characteristic diagram showing the relationship between theimpellent and the displacement in the magnetic circuits according to thepresent invention and prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a magnetic circuit, its manufacturing method andapparatus, an actuator and a speaker each having the magnetic circuitincorporated therein according to the present invention will bedescribed in detail with reference to the accompanying drawings.

First, as regards a permanent magnet incorporated into a yoke member ofa magnetic circuit, not a cylindrical shape, but segments of a shapeinto which the cylindrical shape is divided, i.e. permanent magnetsegments having a partially circular arc in cross section (C-shapedpermanent magnet segments) are used in the present invention.

The direction of orientation of the C-shaped magnet segments may beeither a direction parallel to the width direction (directionsubstantially orthogonal to the partially circular arc cross section)thereof or a direction of the thickness thereof. When it is intended toconverge the magnetic flux so as to contribute to a coil disposed at thecenter portion of an actuator, for example, it is preferred to disposethe segments in the thickness direction. Incidentally, when the C-shapedpermanent magnet segments oriented in the width direction, it ispreferred to increase the number of division of the segments.

In order to conform to the miniaturization of an actuator, for example,the C-shaped permanent magnet segments can be formed into a smallthickness of 2 mm or less. While it is difficult to form a-cylindricalmagnet having a thickness of 2 mm or less from a cylindrical rare earthsintered magnet, when a rare earth sintered magnet is formed intoC-shaped permanent magnet segments, even those having a thickness of 2mm or less are relatively easy to produce. Furthermore, a processingmachine may be used to fabricate C-shaped permanent magnet segments froma rectangular parallelepiped block.

Though the material for the C-shaped permanent magnet segments isoptional insofar as it functions as a permanent magnet, in view of theapplication thereof to an actuator or a speaker high in performance, arare earth sintered magnet proves to be advantageous. The rare earthsintered magnet is composed preponderantly of a rare earth element, atransition metal element and boron. The magnet composition mayoptionally be selected depending on the performance required. When anR-T-B-based rare earth sintered magnet (wherein R stands for at leastone of rare earth elements, T for at least one of transition metalelements containing as an indispensable element or elements Fe or Fe andCo, and B for boron) is adopted, for example, for the purpose ofobtaining a rare earth sintered magnet excellent in magneticcharacteristics, the composition thereof after being sintered ispreferably composed of 20 to 40 mass % of rare earth elements R, 0.5 to4.5 mass % of boron B and the balance of transition metal elements T.Here, R stands for at least one rare earth element selected from thegroup consisting of Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yband Lu. Among other rare earth elements enumerated above, Nd ispreferably used as a principal component because it is resourceful andrelatively inexpensive. In addition, inclusion of Dy is effectivebecause it increases an anisotropic magnetic field to enhance thecoercive force Hcj. Furthermore, it is made possible to add an additiveelement M to form a R-T-B-M-based rare earth sintered magnet. Theadditive element M raised herein is at least one member selected fromthe group consisting of Al, Cr, Mn, Mg, Si, Cu, C, Nb, Sn, W, V, Zr, Ti,Mo, Bi, Ga.

The C-shaped permanent magnet segments are fabricated from a rare earthsintered magnet by compression molding in a magnetic field raw materialalloy powder, such as Nd—Fe—B-based magnetic powder, for example, into aC-shape. The compression molding is performed, with an orientationmagnetic field applied to the direction of the thickness, for example,of a compacted body to be molded. It is made possible to realize asubstantially radial orientation utilizing a special metal mold. Thecompacted body obtained is subjected to heat treatment (sintering andaging treatment) to fabricate C-shaped permanent magnet segments (rareearth sintered magnets). Even in the fabrication of the C-shapedpermanent magnet segments from rare earth sintered magnets, occurrenceof deformation thereof and cracks and chips therein is slight during theprocess of sintering.

The C-shaped permanent magnet segment may have a shape corresponding tothat obtained by dividing a cylindrical magnet into an optional number.When the curvature of the outer circumference (partially circular arc)of the segment is set to be substantially 90°, for example, foursegments are combined to obtain a substantially cylindrical shape.

FIG. 1( a) and FIG. 1( b) illustrate one example of a magnetic circuithaving four permanent magnet segments 1 incorporated in a cylindricalyoke member 4 and an actuator using the magnetic circuit. Theincorporation of the four permanent magnet segments 1 configures asubstantially cylindrical permanent magnet. The inner circumferentialsurfaces 4 a of the four permanent magnet segments 1 are magnetized tohave the same polarity and, when the substantially cylindrical permanentmagnet is configured, all the inner circumferential surfaces have thesame polarity (N pole, for example) and all the outer circumferentialsurfaces have the opposite polarity (S pole, for example). When atransducer 6 equipped with a drive coil 5 is disposed inside thepermanent magnet segments 1 of the magnetic circuit combined in acylindrical form to vibrate the transducer 6 vertically in FIG. 1( b) bymeans of the cooperation of the permanent magnet segments 1 with thedrive coil to configure an actuator. In this actuator, a control device(not shown) controls the electric current flowing through the drive coil5 and controls the actuation of the actuator.

The cylindrical yoke member 4 is provided at one end thereof with acollar 4 b, as shown in FIG. 1( b), which is turned back substantiallyat right angles and on which the bottoms of the permanent magnetsegments 1 are supported. Therefore, the permanent magnet segments 1 aremagnetically attracted to both the inner circumferential surface 4 a andthe collar 4 b of the yoke member 4.

In the magnetic circuit of the present invention, it is important thatthe permanent magnet segments 1 be magnetically attracted to both theinner circumferential surface 4 a and the collar 4 b of the yoke member4, as described above, thereby enabling firm fixation thereof withoutuse of an adhesive agent. With respect to the case where the permanentmagnet segments 1 were magnetically attracted to both the innercircumferential surface 4 a and the collar 4 b of the yoke member 4(Example) and the case where they were magnetically attracted only tothe inner circumferential surface 4 a of the yoke member 4 provided withno collar 4 b (Comparative Example), drop tests were actually conductedto examine the states of fixation of the permanent magnet segments 1.The results thereof are shown in Table 1 below. Incidentally, tensamples of actuators were tested for the movement of the permanentmagnet segments 1 through the visual observation when the samples wereallowed to fall from each of the heights of 30 cm, 100 cm and 150 cm.

TABLE 1 Height 30 cm 100 cm 150 cm Example 0/10  1/10  3/10 ComparativeExample 8/10 10/10 10/10

It was clear from Table 1 above that the number of the permanent magnetsegments moved in each of the ten samples according to the case wherethe permanent magnet segments 1 were magnetically attracted to both theinner circumferential surface 4 a and the collar 4 b of the yoke member4 (Example) was a few at most, whereas almost all of the permanentmagnet segments in each of the ten specimens according to the case wherethe permanent magnet segments 1 were magnetically attracted only to theinner circumferential surface 4 a of the yoke member 4 (ComparativeExample) was moved.

As described above, in the magnetic circuit of the present invention,the intensity of fixation between the yoke member 4 and the permanentmagnet segments 1 can sufficiently be obtained only by means of themagnetic intensity between them to enable the enhancement of theproductivity and the low-cost fabrication. In spite of not requiring useof an adhesive agent, the present invention can provide a magneticcircuit sufficiently endurable to vibration resulting from the drop etc.thereof and not problematic in terms of reliability.

When a magnetic circuit is configured through the incorporation ofplural permanent magnet segments 1, as described above, preferablyattention is paid to the intervals between the adjacent permanent magnetsegments 1. Specifically, the interval is preferably in the range of0.005 to 0.5 mm, more preferably in the range of 0.1 to 0.3 mm. Thisinterval enables the permanent magnet segments 1 to be incorporatedwithout any collision against each other, prevents the magneticintensity thereof from being lowered to a great extent as compared evenwith a ring magnet (radially oriented magnet etc.) and enables thedropping impact to be absorbed. Since the permanent magnet segments 1are magnetically attracted to the yoke member 4 by means of the magneticintensity, the magnetic intensity is converted in the presence of a highimpact into a frictional force. Therefore, the segments can move by theamount of the interval to generate pseudo impact absorption. This is whythe dropping impact can be absorbed.

The magnetic circuit of the above configuration is fabricated by thefollowing procedure. First, the C-shaped permanent magnet segments 1fabricated are magnetized. As shown in FIG. 2, for example, the pluralC-shaped permanent magnet segments 1 are overlapped and subjected tomagnetization treatment in a lump. To realize high performance, themagnetization treatment is performed in the direction of the thicknessof the segments. To magnetize the overlapped C-shaped permanent magnetsegments 1 in the thickness direction, the direction of themagnetization magnetic field H may be set to be a vertical direction inFIG. 2.

While the magnetized C-shaped permanent magnet segments 1 areincorporated into the yoke member to fabricate a magnetic circuit, whenthe C-shaped permanent magnet segments are adopted, as described above,it is required that plural C-shaped permanent magnet segments becombined into a cylindrical shape. In this case, one-by-oneincorporation of the C-shaped permanent magnet segments 1 isinconvenient in workability and makes it difficult to efficientlymanufacture magnetic circuits.

In view of the above, the present invention uses a manufacturingapparatus shown in FIG. 3, for example, to incorporate a required numberof C-shaped permanent magnet segments 1 into a yoke member in a lump.

The manufacturing apparatus shown in FIG. 3 comprises a center rod 2that permits the C-shaped permanent magnet segments 1 to be magneticallyattracted to the peripheral surface thereof and a nonmagnetic supporter3 that is disposed outside the center rod. Since the center rod 2 isrequired to have the C-shaped permanent magnet segments 1 attractedmagnetically thereto, it has to be formed of a magnetic material. Thediameter of the center rod has to be determined so that the curvature ofthe outer circumferential surface may conform substantially to that ofthe inner circumferential surface of the C-shaped permanent magnetsegments 1.

On the other hand, the nonmagnetic supporter 3 assumes a ring(cylindrical) shape and is disposed on the outer circumferential surfaceof the center rod 2 in a manner coaxial with the center rod 2. That isto say, the nonmagnetic supporter 3 is disposed so that it may cover theouter periphery of the center rod 2 inserted into an opening of thesupporter. The center rod 2 and the nonmagnetic supporter 3 are movablerelative to each other. Therefore, the center rod 2 is detachablyinserted into the nonmagnetic supporter 3. The nonmagnetic supporter 3is abutted on the end faces of the C-shaped permanent magnet segments 1and, during the extraction of the center rod 2 that will be describedlater, fulfills a role of regulating the positions of the segments.Therefore, it is formed of a nonmagnetic material so as not to permitthe segments 1 to be magnetically attracted thereto. Incidentally, whenmagnet segments having very high magnetic intensity are used as thepermanent magnet segments 1, since the magnetic attraction thereof tothe center rod 2 is high, a large force is required to extract thecenter rod 2. In this case, the center rod 2 may be provided on thesurface thereof with a separate nonmagnetic material to isolate themagnetic material constituting the center rod and the permanent magnetsegments 1 from each other via the nonmagnetic material, therebyadjusting the magnetic attraction between them.

While the manufacturing apparatus fundamentally has a very simplestructure equipped with the center rod 2 and the nonmagnetic supporter3, it enables plural C-shaped permanent magnet segments 1 to be fixedonto the inner circumferential surface 4 a of a cylindrical yoke member4 in a lump. The operation of inserting the magnet using themanufacturing apparatus will be described below with reference to FIG.4.

The C-shaped permanent magnet segments 1 are inserted into and fixed tothe yoke member 4 according to the following procedure. First, theC-shaped permanent magnet segments 1 are magnetically attracted to thecircumferential surface of the center rod 2 as shown in FIG. 4( a). Inthe case of combining four C-shaped permanent magnet segments 1, thecurvature of the partial circular arc of which is substantially 90°, forexample, into a substantially cylindrical form, the C-shaped permanentmagnet segments 1 are supplied onto the circumferential surface of thecenter rod 2 from four directions, respectively, and magneticallyattracted to the center rod 2 by means of the magnetic intensity of thesegments 1. In this case, the overlapped C-shaped permanent magnetsegments 1 set in a magazine etc., for example, may be extruded tosupply them one by one onto the circumferential surface of the centerrod 2.

FIG. 4( b) shows the state in which four C-shaped permanent magnetsegments 1 have been attracted magnetically to the circumferentialsurface of the center rod 2. When the C-shaped permanent magnet segments1 oriented in their respective thickness direction have been magnetized,for example, the inner circumferential surfaces of all the C-shapedpermanent magnet segments 1 have the same polarity. Though the samepoles are opposed to each other when the segments have been combinedinto a cylindrical shape, the magnetic attraction of the segments to theperipheral surface of the center rod formed of a magnetic materialprevents the opposed segments 1 from inducing magnetic repulsion. Thedescription given here is made with reference to the example in whichthe C-shaped permanent magnet segments 1 of a circular arc in crosssection are incorporated using the columnar center rod 2. This is notlimitative. For example, polygonal permanent magnet segments 1 may beincorporated into the yoke member 4 using a polygonal center rod 2.

After the four C-shaped permanent magnet segments 1 are magneticallyattracted to the peripheral surface of the center rod 2, as shown inFIG. 4( c), the center rod 2 is inserted into the cylindrical yokemember 4 to oppose the C-shaped permanent magnet segments 1 to the innercircumferential surface 4 a of the yoke member 4. The nonmagneticsupporter 3 is then used to regulate the positions of the C-shapedpermanent magnet segments 1 so as not move the segments upward and, inthis state, the center rod 2 is extracted as shown in FIG. 4( d) toattract magnetically and fix to the inner circumferential surface of theyoke member 4 the segments that have been magnetically attracted to theperipheral surface of the center rod 2.

FIG. 5 illustrates in detail the operations shown in FIG. 4( c) and FIG.4( d). FIG. 5( a) shows the state in which the center rod 2 has beeninserted into the cylindrical yoke member 4. In this state, the C-shapedpermanent magnet segments 1 are opposed to the inner circumferentialsurface of the yoke member 4 to exert the magnetic attraction alsobetween the segments 1 and the yoke member 4. At this time, however,since the magnetic attraction exerted between the segments 1 and thecenter rod 2 is larger, the segments 1 are kept attracted magneticallyto the peripheral surface of the center rod 2.

Then, as shown in FIG. 5( b), the nonmagnetic supporter 3 is used toposition-regulate the segments 1 so as not move the segments upward and,in this state, the center rod 2 is elevated. Since the center rod 2 isinsertable into and detachable from the nonmagnetic supporter 3, it ispossible to upwardly move and extract the center rod 2, with thenonmagnetic supporter 3 fixed. At this time, the nonmagnetic supporter 3is abutted on the end faces of the segments 1 to prevent the segments 1from their elevation accompanied by the elevation of the center rod 2.

When the center rod 2 has been extracted as described above, it assumesthe state in which it has been extracted from among the segments 1combined into a cylindrical shape to sharply reduce the magneticattraction exerted between the segments 1 and the center rod 2. When thecenter rod 2 has completely been extracted from among the segments 1,there gives rise to a state in which the magnetic attraction exertedbetween the center rod 2 and the segments has been eliminated. As aresult, the segments 1 are attracted to the inner circumferentialsurface 4 a of the yoke member 4 by means of the magnetic attractionexerted between the segments 1 and the yoke member 4. As shown in FIG.5( c), the segments are magnetically attracted and fixed to the innercircumferential surface 4 a of the yoke member 4. To make the state offixation of the segments 1 to the circumferential surface 4 a morestable, fixation by means of an adhesive agent may also be added.

Incidentally, when four C-shaped permanent magnet segments 1 are to beinserted into and fixed to the yoke member 4, the interval of theadjacent segments 1 disposed on the inner circumferential surface 4 a ofthe yoke member 4 is preferably in the range of around 0.1 to 0.5 mm.When the interval is unduly small, there will be a possibility of thefour segments being inserted into the yoke member at a time only withdifficulty from the standpoint of the dimensional accuracy of thesegments 1. Inversely, when the interval is unduly large, the magneticcharacteristic (a magnetic flux contributing to a coil) will be reducedto possibly lower the impellent of an actuator, for example. In orderfor the segments 1 to be attracted also to the collar 4 b of the yokemember 4, the segments 1 are preferably disposed on the side where thecollar 4 a is formed. By doing so, the firm fixation described above canbe materialized.

As described in the foregoing, since the C-shaped permanent magnetsegments 1 are inserted into and fixed to the yoke member 4 using thecenter rod 2 equipped with the nonmagnetic supporter 3, the cumbersomeinsertion of the magnet into the yoke member can continuously beperformed for a short period of time. The operation at this time isready to make, thereby enabling actuators to be manufactured with highproductivity. When adopting a structure having the center rod 2 ornonmagnetic supporter 3 biased by means of by biasing force of a coilspring etc. and a same mechanism as a knock-type ballpoint pen etc.disposed, a one-push operation enables the insertion of a magnet.Furthermore, since the C-shaped permanent magnet segments 1 aremagnetized in advance in a lump, it is unnecessary for the segments 1 tobe magnetized after the incorporation thereof into the yoke member 4.This also enables the productivity to be improved.

In the manufacturing apparatus, when adopting a structure in whichplural center rods 2 each having a nonmagnetic supporter 3 are arrayedand magnet insertion into plural yoke members 4 is carried out at thesame time, a high efficiency can be attained. FIG. 6 shows one exampleof the manufacturing apparatus of a configuration having four centerrods 2 each having a nonmagnetic supporter 3, disposed and the magnetinsertion into four yoke members 4 is carried out at the same time. Theyoke members 4 are set in position in a carrier pallet 7 having circularconcave portions formed in conformity with the shape of the yokemembers, and the magnet insertion is performed in the aforementionedoperation. FIG. 6( a) shows the state in which the C-shaped permanentmagnet segments 1 have been attached to the center rod 2 and FIG. 6( b)the state in which the segments 1 have been attached to the yoke members4.

Incidentally, in this example, the center rod 2 has a leading end 2 aserving as a guide for guiding the center rod 2 into the yoke member 4and thus it is possible to accurately position the center rod 2 relativeto the yoke member 4. When it is intended to perform magnet insertionrelative to the plural yoke members 4 at the same time, the positionalaccuracy of each center rod 2 relative to each yoke member 4 isrequired. The utilization of the leading end 2 a as a guide enablesaccurate magnet insertion into the yoke members 4.

In the configuration of the manufacturing apparatus as shown in FIG. 6,the magnet insertion relative to the four yoke members 4 can beperformed in a lump and, at the same time, the magnet segments can besupplied in turn to the yoke members 4 set in position in the carrierpallet 7. Thus, the treatment can continuously be made to enhance theefficiency to a great extent.

Use of the manufacturing method and manufacturing apparatus according tothe present invention enables incorporation of plurally divided C-shapedpermanent magnet segments 1 into the yoke member 4 with ease and enablesmanufacture of high-performance miniaturized magnetic circuits with highefficiency. Furthermore, since not a cylindrical permanent magnet, butthe C-shaped permanent magnet segments 1 are incorporated, it ispossible to eliminate the difficulty in manufacturing a cylindricalpermanent magnet, particularly a cylindrical rare earth sintered magnetand manufacture magnetic circuits with high productivity at low cost.

The magnetic circuit, manufacturing method and manufacturing apparatusaccording to the present invention are not limited to the examplesdescribed above. In accordance with the shape etc. of the yoke member 4,for example, various changes can be made. Modifications of the magneticcircuit, manufacturing method and manufacturing apparatus according tothe present invention will be described hereinafter.

FIG. 7( a) shows a case where the yoke member 4 having the turned-backcollar 4 b formed on the inside diameter part thereof is provided with aturnup part 4 c turned up further at right angles. When it is intendedto attach and fix the C-shaped permanent magnet segments 1 to the yokemember 4 of this shape, generally it is very difficult to do so becausethe turnup part 4 c constitutes an obstacle to the insertion of thesegments.

In such cases as this, utilization of the manufacturing apparatusaccording to the present invention enables magnet insertion readily andinfallibly, provided that the center rod 2 has to be formed into ahollow pipe to permit the insertion of the turnup part 4 c into thehollow part thereof. This shape of the center rod 2 enables simultaneouspositioning of the center rod 2 relative to the yoke member 4 andaccurate magnet insertion. With the same apparatus configuration andsame operation method as in the embodiment shown in FIG. 3 to FIG. 5,except for the formation of the center rod 2 into the hollow pipe, it ismade possible to attach the C-shaped permanent magnet segments 2 to theyoke member 4.

FIG. 7( b) shows an example in which the C-shaped permanent magnetsegments 1 are attached to an outer circumferential surface 4 d of theyoke member 4. In this case, the center rod 2 and the nonmagneticsupporter 3 are formed into a cylindrical form, and the nonmagneticsupporter 3 is disposed on the inner circumferential surface of thecenter rod 2. In addition, in the magnet insertion, the C-shapedpermanent magnet segments 1 are magnetically attracted to the innercircumferential surface of the center rod 2. This can be achievedperforming the operation similar to that utilizing the center rod andnonmagnetic supporter shown in FIG. 3 or FIG. 4 and setting the segments1 in position on the inner circumferential surface of the cylindricalyoke member. The other operating method is the same as that shown inFIG. 3 to FIG. 5.

FIG. 7( c) shows a modification on the shape of the yoke member 4. Thestructures of the center rod 2 and nonmagnetic supporter 3 and theoperation method of the magnet insertion are the same as those of theembodiment shown in FIG. 3 to FIG. 5. In the present modification, theyoke member 4 is provided at the outer peripheral bottom with a concavepart 4 e in which an adhesive agent is to be filled. This structureenables the C-shaped permanent magnet segments 1 to be magneticallyattracted and fixed to the inner circumferential surface of the yokemember 4 and also fixed simultaneously by means of the adhesive agentfilled in the concave part 4 e.

The structure of the yoke member of this embodiment bending-processedfrom magnetic metal plate, such as of iron, is advantageous in terms ofthe enhancement of the accuracy of the attachment positions of theC-shaped permanent magnet segments 1. When a magnetic metal plate isbending-processed, the corner portion thereof is inevitably formed intoa slight R-shape and, when the C-shaped permanent magnet segments 1 areto be attached and fixed thereto, these have to be disposed slightlyupward from the bottom thereof. The formation of the concave part 4 eenables the R-shaped portion to be set back and the C-shaped permanentmagnet segments 1 to be infallibly in contact with the innercircumferential surface 4 a and the collar 4 b of the yoke member 4simultaneously. Thus, the C-shaped permanent magnet segments 1 canmagnetically be fixed firmly to the yoke member 4. While the concavepart 4 e is formed on the bottom in FIG. 7( c), it may be formed on theside of the circumferential surface of the yoke member.

In the manufacturing method and apparatus of the present invention, themethod of supplying the C-shaped permanent magnet segments 1 to thecenter rod 2 can be modified. While the present embodiment intends todisposed magazines in the four direction of the center rod 2 and supplyfour C-shaped permanent magnet segments 1 from the magazines to theperipheral surface of the center rod 2, since the segments aremagnetically attracted to the center rod 2 with great force by themagnetic attraction, there is a possibility of a disadvantage beingincurred, such as a damage inflicted on the segments due to theirrespective collision. In addition, in many cases, the segments 1 arestored as accommodated in individual cavities for the purpose ofpreventing damage and contamination thereof, the procedure of resettingthe stored segments in position in the magazines is made cumbersome andcomplicated.

To eliminate these disadvantages, the configuration shown in FIG. 8, forexample, may be adopted, in which a magnet-setting alignment guide 9 isattached to each of center rods 2, and C-shaped permanent magnetsegments 1 are inserted into openings 9 a of the guides 9 and set inposition one by one on the circumferential surfaces of the center rods2.

The setting process will be described in detail. First, themagnet-setting alignment guide 9 is attached to each of the center rods2. In this case, the direction in which the center rod 2 is installed ismade horizontal unlike the vertical direction in the precedingembodiment, and the yoke members 4 that are set in position in thepallet 7 are directed horizontally.

The C-shaped permanent magnet segments 1 are accommodated in amagnetizing jig 8 having individual cavities 8 a formed as predeterminedconcaves, taken out one by one and set in position on thecircumferential surface of the center rod 2. The segments 1 can easilybe taken out from the individual cavities 8 a using a pickup jig (notshown) having a metal chip attached to the leading end thereof. Thesegment 1 taken out with the pickup jig is magnetically attracted to theperipheral surface of the center rod 2, with the segmentposition-aligned utilizing the opening 9 a of the magnet-settingalignment guide 9.

FIG. 9 shows a procedure of successively attaching the C-shapedpermanent magnet segments 1. To attach a C-shaped permanent magnetsegment 1 to the peripheral surface of the center rod 2, as shown inFIG. 9( a), a first segment 1 is magnetically attracted and fixed to theperipheral surface of the center rod 2 at a predetermined positionutilizing the opening 9 a of the magnet-setting alignment guide 9. Atthis time, when four projections 2 b are provided on the peripheralsurface of the center rod 2 as boundaries of the segments 1, thesegments can be attached to the peripheral surface in a more accuratelypositioned state.

After the first segment 1 is attached to the center rod 2, the rod isrotated counterclockwise to move the first segment 1 as shown in FIG. 9(c). When four segments 1 are to be combined, the angle of rotation isapproximately 90°. The direction of rotation may either becounterclockwise or clockwise.

Then, as shown in FIG. 9( c), a second segment 1 is magneticallyattracted and fixed to the peripheral surface of the center rod at apredetermined position utilizing the opening 9 a of the magnet-settingalignment guide 9. By repeating this procedure four times, theattachment of the four segments 1 to the center rod is completed. Themagnet insertion into the yoke 4 is the same as in the precedingembodiment, provided that the direction of the insertion and extractionis horizontal.

According to the present embodiment, it is unnecessary to set theC-shaped permanent magnet segments 1 in position from the magnetizingjig 8 into magazines and possible to supply the segments 1 from themagnetizing jig 8 directly onto the center rod 2. Since the segments 1are attached to the center rod one by one, it is possible to avoid anydamage on the segments due to their collision.

The magnetic circuit, its manufacturing method and apparatus andactuator having the magnetic circuit incorporated therein according tothe present invention have been described in the foregoing. The magneticcircuit can be used for a speaker as well besides the actuator.

FIG. 10 illustrates an example of a speaker using the magnetic circuitof the present invention. A magnetic circuit portion 11 fundamentallyhas the same configuration as the magnetic circuit described above, inwhich plural permanent magnet segments 13 are magnetically attracted tothe inner wall of a cylindrical yoke member 12. Here, the segments 13are magnetized so that the inner circumferential surfaces thereof havethe same polarity. When the segments 13 are combined, they aremagnetically attracted to the inner wall of the yoke member 12, withtheir opposed surfaces exhibiting the same polarity. The cylindricalyoke member 12 comprises an outer cylinder portion 12 a, a bottomportion 12 b turned back at right angles at the bottom of the outercylinder portion 12 a and an inner cylinder portion 12 c turned up atright angles from the bottom portion 12 b. The inner cylinder portion 12c faces the segments 13, leaving a predetermined space between them as amagnetic gap.

In the speaker of this embodiment, a voice coil bobbin 15 having a voicecoil 14 wound around it is inserted into the magnetic gap to inducevibration of the speaker in the vertical direction in FIG. 10 throughthe cooperation of the voice coil 14 and the magnetic circuit portion 11(permanent magnet segments 13).

A diaphragm 16 is joined to the voice coil bobbin 15 and vibrated withthe operation of the voice coil bobbin 15. The voice coil bobbin 15 isjoined to the center of the diaphragm 16 and also joined via a damper 17to a frame 18.

The diaphragm 16 in this embodiment is a cone-shaped one and has itsouter circumferential edge joined to the frame 18. The center portion ofthe diaphragm 16 is provided with a dust cap 19 for preventing dust fromentering the magnetic circuit portion 11 and voice coil bobbin 15.Incidentally, the diaphragm 16 is not limited to the cone-shaped one,but any diaphragm of an optional shape may be adopted. A concreteexample thereof is a tabular diaphragm having a honeycomb structure. Thematerial for the diaphragm 16 is optional and includes cone paper,plastic, carbon fiber and their combinations, and any of theconventional diaphragm materials is available.

In the speaker of the present embodiment, the voice coil bobbin 15 isdisposed so as to face the permanent magnet segments 13 disposed into acylindrical shape. When adopting this type of magnetic circuit, use offerrite magnets as the segments 13 is insufficient because the ferritemagnets have a small magnetic intensity to possibly fail to obtain adesirable vibration stroke. Therefore, when adopting the magneticcircuit for the speaker, rare earth sintered magnets are preferably usedas the permanent magnet segments 13.

FIG. 11 shows a difference in impellent between voice coil bobbins whenusing different magnetic circuits. When the permanent magnet segments 13are disposed into a cylindrical shape, has their inner circumferentialsurfaces facing the voice coil 14 as in the present embodiment and arefurther formed of rare earth magnets (NdFeB-based magnets), it ispossible to obtain stable impellent in a wide range of displacement asshown by line “a” in FIG. 11. On the other hand, when using an ordinarymagnetic circuit having a ring-shaped ferrite magnet, while higherimpellent can be obtained in case where the displacement is small, theimpellent is rapidly lowered in case where the displacement becomeslarge. A speaker is required to have a large vibration stroke to attaina high phonetic quality. Adoption of the magnetic circuit of the presentinvention can satisfy the requirement.

1. A magnetic circuit comprising: a cylindrical yoke member having aninner circumferential surface and a collar formed at one end thereof andturned back substantially at right angles; and a plurality of permanentmagnet segments of a partially circular arc in cross section havingrespective outer surfaces attracted magnetically to the innercircumferential surface of the cylindrical yoke member and respectiveone end faces attracted magnetically to the collar of the cylindricalyoke member.
 2. A magnetic circuit according to claim 1, wherein thepermanent magnet segments are disposed at intervals of 0.05 mm to 0.5mm.
 3. A magnetic circuit according to claim 1, wherein the permanentmagnet segments are disposed at intervals of 0.1 mm to 0.3 mm.
 4. Amagnetic circuit according to claim 1, wherein the permanent magnetsegments are formed of a rare earth sintered magnet.
 5. A method for themanufacture of a magnetic circuit, comprising the steps of: magneticallyattracting a permanent magnet segment of a partially circular arc incross section to an outer peripheral surface of a center rod; causingthe magnet segment in a magnetically attracted state to face a yokemember; using a nonmagnetic supporter to regulate the magnet segment inposition; extracting the center rod from the magnet segment in aposition-regulated state; and magnetically attracting and fixing themagnet segment to the yoke member.
 6. A method for the manufacture of amagnetic circuit according to claim 5, wherein the yoke member has acylindrical shape and wherein the magnet segment is magneticallyattracted and fixed to an inner circumferential surface of thecylindrical yoke member.
 7. A method for the manufacture of a magneticcircuit according to claim 6, wherein the magnet segment comprises aplurality of magnet segments that are disposed to form a substantiallycylindrical shape on the outer peripheral surface of the center rod andattracted and fixed in a lump to the inner circumferential surface ofthe yoke member.
 8. A method for the manufacture of a magnetic circuitaccording to claim 7, wherein the magnet segments are supplied to theouter peripheral surface of the center rod, respectively, from differentdirections.
 9. A method for the manufacture of a magnetic circuitaccording to claim 7, wherein the magnet segments are supplied to theouter peripheral surface of the center rod from a same direction, withthe center rod rotated, thereby attracting the magnet segmentssequentially to the outer peripheral surface of the center rod.
 10. Amethod for the manufacture of a magnetic circuit according to claim 5,wherein the permanent magnet segment is magnetized in advance in adirection of a thickness thereof.
 11. An apparatus for the manufactureof a magnetic circuit, comprising: a center rod formed of a magneticmaterial and having an outer peripheral surface to which a permanentmagnet segment of a partially circular arc in cross section isattracted; and a nonmagnetic supporter assuming a ring shape anddisposed around the outer peripheral surface of the center rod andcoaxially with the center rod; wherein the center rod is insertable intoand detachable from the nonmagnetic supporter.
 12. An apparatus for themanufacture of a magnetic circuit according to claim 11, furthercomprising means for supplying the magnet segment to the outerperipheral surface of the center rod.
 13. An apparatus for themanufacture of a magnetic circuit according to claim 12, wherein thesupplying means comprises a plurality of supplying means disposed toface the outer peripheral surface of the center rod.
 14. An apparatusfor the manufacture of a magnetic circuit according to claim 11, furthercomprising a magnet setting alignment guide provided with an openingcorresponding to a shape of the magnet segment and attached to thecenter rod.
 15. An actuator comprising a magnetic circuit and a drivecoil that cooperate with each other to generate an actuation force, themagnetic circuit comprising a cylindrical yoke member having an innercircumferential surface and a collar formed at one end thereof andturned back substantially at right angles; and a plurality of permanentmagnet segments of a partially circular arc in cross section havingrespective outer surfaces attracted magnetically to the innercircumferential surface of the cylindrical yoke member and respectiveone end faces attracted magnetically to the collar of the cylindricalyoke member.
 16. An actuator according to claim 15, wherein thepermanent magnet segments are formed of a rare earth sintered magnet.17. A speaker comprising a magnetic circuit, a voice coil and adiaphragm that is joined to the voice coil and vibrated by means of themagnetic circuit and the voice coil in cooperation with each other, themagnetic circuit comprising a cylindrical yoke member having an innercircumferential surface and a collar formed at one end thereof andturned back substantially at right angles; and a plurality of permanentmagnet segments of a partially circular arc in cross section havingrespective outer surfaces attracted magnetically to the innercircumferential surface of the cylindrical yoke member and respectiveone end faces attracted magnetically to the collar of the cylindricalyoke member.
 18. A speaker according to claim 17, wherein the permanentmagnet segments are formed of a rare earth sintered magnet.